Babies lear to talk by "computing" sounds

One of the hardest things in understanding a new language is to
pick out words from the rushing stream of speech. Babies master
this challenge around the age of 8 months and do it, surprisingly
enough, by thinking like little statisticians, according to a
University study, published in the December issue of Science.

The study, authored by Jenny R. Saffran, a doctoral student in
Brain and Cognitive Sciences, and co-authored by Professors Richard
N. Aslin and Elissa L. Newport, offers powerful evidence that
infants are extraordinary learners. After hearing only a two-minute
sample of speech, babies speedily detect clear patterns in the
sounds of language. Such vigorous "computational abilities"
can help explain how very young humans learn so much about their
world so quickly, the authors say.

(In a second article called "Learning Rediscovered"
in the same issue of Science, two UC San Diego psychologists
explain that the Rochester study is an important new argument
for the "learning" side of the ledger in the long
scholarly debate over whether language is "innate"
or "learned.")

Linguists have known for years that recurrence of sound patterns
plays some role in learning language. Consider the four-syllable
phrase, "pretty baby." Because "pretty"
is a word, the first sound "pre" is often followed
by the second sound, "ty." Similarly, the sound "ba"
is often followed by the sound "by." However, the
pattern "tyba," consisting of the end of one word
and the beginning of another, is less common. As babies hear language,
they become aware that sound combinations like "pretty"
and "baby" occur more frequently than other sound
combinations like "tyba."

The new study shows that babies can use this statistical information
alone to learn where one word ends and another begins, according
to Saffran.

"In real life, babies use lots of cues to tell what's
a word and what isn't," said Saffran. "They
listen to pauses, to changes in pitch, stress and rhythm to figure
it out. But we wanted to see if babies could learn using only
statistical information, so we made it really tough on them: We
made up a nonsense language, and took out all the cues except
statistics to see if babies could still learn. Amazingly, they
did."

To test whether babies could extract individual words from continuous
speech by relying upon statistical information alone, the authors
designed a study using a nonsense language spoken by a voice synthesizer.
The synthesizer produced flat, monotone speech with no pauses
between words. Twenty-four 8-month-old infants participated.

To familiarize the babies with the nonsense language, the investigators
played a two-minute sample of four nonsense words, repeated in
random order. The speech sounded something like the following:
"bidakupadotigolabubidaku...."

Then, to assess what the babies had learned, the investigators
let the babies listen to either words from the language ("bidaku
bidaku bidaku...") or sequences that the babies had heard
which were not words ("dakupa dakupa dakupa....")
These "part-word" sequences spanned word boundaries,
like "tyba" in the "pretty baby" example.

In order to see what the babies had learned, the investigators
took advantage of a simple fact about infants. "As every
parent knows, babies are restlessly curious and get bored quickly,"
Saffran says. "They'll explore a new toy longer
than one that's been around them all the time, so if you
want to hold their attention longer, you need to keep coming up
with something new and fresh. We thus asked whether the infants
would listen longer to the part-words, because if the infants
had learned and remembered the words from the two minute speech
stream, the part-words should seem relatively new and interesting."

By keeping track of the amount of time infants listened to the
words and part-words, the investigators saw clear evidence that
babies recognized the difference between the words and the part-word
sequences crossing word boundaries. The babies listened to the
unfamiliar part-word sequences longer than the now-familiar words
- demonstrating, as infants will, a longer attention span for
something new.

The investigators don't claim that the babies understood
the words in the way adults would--as being a collection
of sounds attached to meaning. Instead, their study shows that
after only two minutes of hearing nonsense words, 8-month-old
infants can remember familiar sequences of sounds, and distinguish
them from unfamiliar ones, and that they can make these distinctions
based on statistical recurrence alone--without the help
of other normal cues in speech, such as pauses, or changes in
pitch or inflection.